The invention concerns generally the technology of generating an input signal to an electronic device in response to a manual operation of a user. Especially the invention concerns a structure that allows a device manufacturer to freely design the outlook of a keyboard or the like.
Keyboards or -pads are conventionally used for converting a manual action of a human user to an input signal comprehensible to a digital signal processing device. FIG. 1 illustrates a known rectangular keypad that consists of Nxc3x97M keys; here N=4 and M=5. The arrangement comprises N conductive input rows 101 to 104 and M conductive output columns 105 to 109 where normally the rows and columns do not have a galvanic contact with each other. At each junction of a row and a column there is a key, comprising a conductive dome shown as a dotted circle and supported over the junction so that normally it does not contact the intersecting conductors. Pressing for example the key shown as 110 will cause the conductive dome to produce a galvanic contact between row 104 and column 109. An input voltage distributor 111 energises each of the input rows in cyclical turns at a rate that is expected to exceed the rate of pressing the keys by the user. An output detector 112 detects the energisation caused in one of the output lines when the energisation of the corresponding input line coincides with a conductive dome coupling the input and output lines at one intersection. The logical encoding block 113 converts the observation of a keystroke to a bit sequence that unequivocally indicates, which key was pressed.
The drawbacks of the arrangement of FIG. 1 comprise the relatively high power dissipation of the N parallel voltage buffers included in block 111 that arc required to maintain the continuous sequential energisation of the input rows, as well as the electromagnetic interference which the rapid cyclical energisation of the rows tends to cause to any nearby sensitive electronic devices.
From the European patent publication number EP 695 041, which is incorporated herein by reference, there is known a keypad arrangement according to FIG. 2. A supply voltage Vcc is provided to a positive voltage rail 201 through a resistor 202. From the positive voltage rail there is a connection to each column of keys through a resistor 203-206 so that each of the resistors has a different value. Each row of keys is coupled to a ground rail 207 so that from each row there is a connection to a common connection point through a differently valued resistor 208-212, and from the common connection point there is a connection to the ground rail through a resistor 213. The positive voltage rail 201 is coupled to a first output port 214. The circuit comprises also a PNP transistor 215 the emitter of which is coupled to the supply voltage Vcc, the base of which is coupled to the positive voltage rail 201 through a resistor 216 and the collector of which is coupled to the ground rail 207 through a resistor 217. The collector of the PNP transistor 214 is also coupled to a second output port 218.
The idea of the keypad according to FIG. 2 is that when a certain key is pressed, a resistive current path is produced between the positive voltage rail and the ground rail. The resistance value of the current path is unambiguously associated to the depressed key, because all the column resistors 203-206 and row resistors 208-212 have different values. The variable resistance of the current path acts as a part of a voltage divider together with the resistor 202, which in turn causes the positive voltage rail to assume a voltage value that is also unambiguously associated to the key which was pressed. An A/D converter (not shown) connected to the first output port converts the voltage value of the positive voltage rail into a digital word representing the depressed key. The circuit built around the PNP transistor will generate an interrupt signal that is readable from the second output port and indicates when a key is pressed.
The keypad of FIG. 2 is much more advantageous than that of FIG. 1 in numerous ways: for example, it causes much less electromagnetic interference and dissipates much less power during operation. However, there are still certain drawbacks. A manufacturer of electronic devices will often want to put a series of slightly different models on the market. In many cases it is most advantageous to keep the inner structures the same from one model to another to as large an extent as possible, and use visible design features to distinguish the models from each other. If the different models have differently located keys in the keypad, or a different number number of keys, the resistor networks and the the printed circuit board that acts as the support structure of the keypad must all be specific to each model. It would be very advantageous if the arrangement of keys could be variable without modifications to the inner structures of the keypad.
It is therefore an object of the present invention to provide a keypad structure that allows for the location and number of keys to be varied without essential modifications to the inner structyres of the keypad. It is also an object of the invention to provide a keypad that is easily adapted to mass production. A further object of the invention is to provide a keypad with a simple and reliable structure.
The objects of the invention are achieved by building the keypad on top of certain essentially continuous conducting means for providing a very large number of unambiguously identifiable current paths as a response to depressions at various locations on the keypad.
The keypad according to the invention is characterised in that it comprises an arrangement of essentially continuous conducting means next to the depressable keys and
connection creating means associated with each depressable key for providing, as a response to the depression of a key, an electrical connection through said essentially continuous conducting means so that the electrical characteristics of the created electrical connection uniquely identify the location of the depressed key in relation to said essentially continuous conducting means.
The invention applies also to an electronic device with a keypad, an analogue to digital converter and some digital circuitry. It is characteristic to the electronic device that
the keypad comprises an arrangement of essentially continuous conducting means next to the depressable keys and connection creating means associated with each depressable key for providing, as a response to the depression of a key, an electrical connection through said essentially continuous conducting means so that the electrical characteristics of the created electrical connection uniquely identify the location of the depressed key in relation to said essentially continuous conducting means, and
the analogue to digital converter is arranged to identify the electrical characteristics of the created electrical connection and to provide digital information that uniquely identifies the depressed key to the digital circuitry.
According to the invention the keypad structure comprises, under the part(s) that consitutes the actual keys, an arrangement of essentially continuous conductive means over a certain area. The conductive means are arranged so that in an idle state (no keys being pressed) there are essentially no current paths through them. The depression of a key creates a temporary current path through the continuous conductive means at a certain location. The electrical characteristics associated with such a temporarily created current path are unique to (fiat location. By sensing the electrical characteristics of the created current path it is possible to identify the location where a key was depressed.
The essential difference between the invention and most prior art keypad structures is that the arrangement of essentially continuous conductive means is equally responsive to depressions at a very large number of locations, whereas the prior art keypad structures have involved a discrete and relatively small number of intersections between columns and lines where the creation of a current path has been possible. The invention enables the designer of the keyboard to place an almost arbitrary number of keys into arbitrary locations. The most important factor that limits the number of keys by setting a minimum distance requirement between adjacent keys is the sensitivity of the sensing means that are used to sense the electrical characteristics of the created current path. In many cases an important limiting factor is also the physical size of the keys; if the keys are meant to be operable by fingers, they should not be so small and so close to each other that an adequate user would find it difficult not to depress two adjacent keys simultaneously.
An advantageous form for the arrangement of essentially continuous conductive means is a meandering pair of elongated resistive strips that cover the whole area over which the keys can be located. Each key will then be associated with a conductive dome that in the idle state does not touch the resistive strips. The depression of a key will cause the respective conductive dome to press against the resistive strips creating a conductive bridge between the strips. By measuring the resistance of the current path that consists of the parts of the resistive strips up to the connection point it is possible to determine, how far from the beginning of the resistive strips was the depression point.
Another advantageous form for the arrangement of essentially continuous conductive means is a pair of conductive planes situated in parallal to each other but not touching each other in the idle state. A depression of a key will create a local conductive contact between the planes at a certain location. By using an arrangement of input and output electrodes along the sides of the planes it is possible to determine the electrical characteristics of a current path that runs through the connection point. From the measured electrical characteristics it is possible to calculate the position of the connection point.
It is possible to integrate into the keypad arrangement according to the invention a circuit for generating an interrupt signal at the depression of a key. Such circuits are well known in the art from e.g. the previously mentioned European patent publication number EP 695 041.